Silica, an inorganic material having silicon dioxide (SiO.sub.2) as a basic structural unit, is useful in a wide variety of commercial applications. Silica exists in a variety of molecular forms, which include, for example, monomers, dimers, oligomers, cyclic forms, and polymers. In addition, silica can be amorphous, crystalline, hydrated, solvated, or dry, and can exist in a variety of particulate and aggregation states.
Amorphous silica can be formed by molecular precipitation, for example, by cooling a supersaturated solution, concentrating an undersaturated solution, or by careful hydrolysis of a solution of a labile silica precursor, such as a SiCl.sub.4, esters of silica, SiS.sub.2, Si(OR).sub.4, and the like, to provide a supersaturated solution of Si(OH).sub.4, from which precipitates amorphous silica.
Pyrogenic, or "fumed silica", which has a particle size from about 2-20 nm, is formed from the vapor phase. For example, silica (usually sand) can be vaporized at about 2000.degree. C. and cooled to form anhydrous amorphous silica particles. Alternatively, silica can be sublimed at about 1500.degree. C. in the presence of a reducing agent (e.g., coke) to form SiO, which can be oxidized to form particulate silica. Other methods of producing fumed silica include, for example, oxidation of SiCl.sub.4 at high temperatures or burning SiCl.sub.4 in the presence of methane or hydrogen.
Silica solutions exhibit polymerization behavior, resulting in the increase of Si--O--Si bonds and decrease of Si--OH bonds. In an aqueous medium, amorphous silica dissolves (and/or depolymerizes), forming Si(OH).sub.4, which undergoes polymerization to form discrete particles with internal Si--O--Si bonds and external Si--OH bonds on the particle surface. Under certain conditions, the polymeric silica particles thus formed will further associate to give chains and networks comprising the individual particles.
Generally, under neutral or alkaline conditions (pH 7 or greater), the particles tend to grow in size and decrease in number, whereas under acidic conditions (pH&lt;7), the particles have a greater tendency to agglomerate to form chains, and eventually three dimensional networks. If salts are present which neutralize the charge produced on the particle surface, agglomeration of particles will be more likely to occur under neutral or alkaline conditions.
The term "sol" refers to a stable dispersion of discrete, colloid-size particles of amorphous silica in aqueous solutions. Under the proper conditions, sols do not gel or settle even after several years of storage, and may contain up to about 50% silica and particle sizes up to 300 nm, although particles larger than about 70 nm settle slowly. A sol can be formed, for example, by growing particles to a certain size in a weakly alkaline solution, or by addition of dilute acid to a solution of sodium silicate (e.g., Na.sub.2 SiO.sub.3) with rapid mixing, until the pH drops to about 8-10, followed by removal of Na.sup.+ (e.g., by ion-exchange resin or electrodialysis). Silica sols, depending upon the type of silica, the particle size, and the nature of the particles, can form gels under mildly acidic to strongly acidic conditions.
The term "gel" refers to a coherent, rigid, continuous three-dimensional network of particles of colloidal silica. Gels can be produced by the aggregation of colloidal silica particles (typically under acidic conditions when neutralizing salts are absent) to form a three dimensional gel microstructure. Whether a gel will form under a particular set of conditions, however, can depend on the silica properties, such as, for example, particle size and the nature of the particle surface. The term "hydrogel" refers to a gel in which the pores (spaces within the gel microstructure) are filled with water, Similarly, the term "alcogel" refers to a gel in which the pores are filled with an alcohol. When a gel is dried (liquid removed from the pores) by means in which the coherent gel microstructure collapses (e.g., by solvent evaporation), a relatively high density collapsed powder, or "xerogel", is formed. In contrast, when a gel is dried by means in which the gel microstructure is preserved (e.g., supercritical drying as described in U.S. Pat. No. 3,652,214), a low density "aerogel" is formed. Silica aerogels have very unusual and highly desirable properties such as, for example, optical transparency, extremely low density, and unprecedented low thermal conductivity. See Herrmann et al., Journal of Non-Crystalline Solids, 186, 380-387 (1995). Silica aerogels are useful in a wide variety of applications which include, for example, thermal insulators and reinforcing fillers for elastomers. Although raw material costs are very low, economically feasible processes for producing aerogels have been pursued unsuccessfully for decades.
It is known that the hydrophobic surface modification of silica can dramatically improve silica properties for use in numerous valuable commercial applications. U.S. Pat. No. 3,015,645 ("Tyler") discloses that hydrophobic silicon powders prepared by reacting silica with hydrophobing agents, such as organosilicon halides, can produce superior reinforcing fillers for siloxane elastomers. U.S. Pat. No. 3,122,520 ("Lentz") discloses an improved method of preparing Tyler-type organosilated silica fillers, which improvement involves subjecting a silica sol to a strong acid at low pH (less than 1) and high temperatures prior to introducing the hydrophobing agent. U.S. Pat. No. 2,786,042 ("Iler") discloses hydrophobic organic surface-modified silica, organosols, and preparation methods thereof.
Although strongly acidic conditions purportedly improve the surface modification of silica, there are serious drawbacks to using strong acids in the production of low density organic-modified silica, particularly when the modifying agent is a reactive organosilane. When trimethylchlorosilane (TMCS) is used as the organic modifying reagent, the formation of undesirable reaction by-products can be controlled to a certain extent under strongly acidic conditions. However, when lower cost di- or tri-functional organosilane modifying agents such as dimethyldichlorosilane (DMDCS) or methyltrichlorosilane (MTCS) are used under strongly acidic conditions, they tend to form by-products, polymerize, and/or crosslink with other silanes, resulting in a higher density product, which is undesirable. Such side reactions can result in poor product quality, low yield, and inefficient utilization of di- and tri-functional organosilane surface modifying agents. These shortcomings pose a substantial barrier to the commercialization of processes related to the production of surface-modified silica with di- and tri-functional organosilane surface modifying agents.
In view of the foregoing problems, there exists a need for an improved method for the surface modification of silica using di- and tri-functional organosilane surface modifying agents. The present invention provides such a method. These and other advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.